As we wish to decrease the contrast more the further away you get, we want the lightest areas in the background. To achieve this, we'll invert the layer by right-clicking it, choosing Effect, Channel and Invert (Fig.14).

Fig. 14

Then set the blending mode to Add (Fig.15).

Fig. 15

We have now successfully put some "air" between the foreground and the background, but in reality, this "air" isn't white on a sunny day, but the colour of the sky. We'll apply a colour correction to the Z-Depth pass by right-clicking it again and choosing Effect, Color Correction and Hue/Saturation (Fig.16).

Fig. 16

Check the box called Colorize and then turn the Colorize Hue knob to somewhere around -160 (Fig.17).

Fig. 17

Also, the sky has turned white, due to the fact that the Z-Depth pass does not contain an alpha channel. With the Z-Depth pass selected in the time line, highlight the Effect Controls panel above and press Ctrl+V/Cmd+V to paste in the matte from earlier. The Take Matte From Layer drop down resets for some reason, so we have to select our matte layer again (Fig.18).

Fig. 18

Step 7: Sky Replacement

Now that we have a basic composite for the buildings, we'll proceed with replacing the sky. First step is to find a suitable image. Personally, I went over to CG Textures (http://cgtextures.com/) and downloaded one of their Skies 360 images. Import the image by pressing Ctrl+I/Cmd+I and pointing to it on your hard drive (Fig.19).

Fig. 19

Next, drag the new sky layer into the composite, directly underneath your main layer (Fig.20).

Fig. 20

Right away, the sky looks too large and rather boring so we'll try making it a bit more interesting. First, reduce its size by expanding the layer (press the little triangle) and scale it down to around 40% under Transform (Fig.21).

Fig. 21

In the viewer, drag the sky layer upwards to match the horizon in the cityscape (Fig.22).